Method and apparatus for making potentiometer windings

ABSTRACT

A method and apparatus for making potentioneter windings whereby the potentiometer wire is wound on an insulated mandrel and thereafter a mixture of adhesive and solvent is sprayed over the wound mandrel from one side of the mandrel. By controlling the spray, the adhesive may be limited to approximately 180* of the mandrel periphery, thereby securing the potentiometer wire to the mandrel over approximately half the mandrel area while leaving clean and uncovered the areas of the mandrel which will become the wiper track and the areas in which end connections and taps will be attached. The spraying process may be accomplished as a separate operation after winding the potentiometer wire on the mandrel, or as a continuous process by an addition of the apparatus of the invention to a suitable winding machine. The apparatus of the invention adapts a conventional artists air brush for use as a sprayer and provides all adjustments and controls necessary to achieve the desired result.

United States Patent 1191 [111 3,791,901 Beauregard Feb. 12, 1974 [54] METHOD AND APPARATUS FOR MAKING 2,647,976 4/1953 Kasten et a1 338/264 POTENTIOMETER WINDINGS 2,620,990 12/1952 Cary et a1. t. 242/703 2,287,460 6/1942 Wagenhals et a1.... 29/618 Inventor: Lmus Beauregard, North 3,477,056 11/1969 Pedu 338/143 Hollywood, Calif. [73] Assignees Techno-Components Corp., Van Primary Emmi"e' Damel Fmsch N C 1'f. 57 ABSTRACT [22] F'led: 1971 A method and apparatus for making potentioneter [21] Appl. No.: 117,652 windings whereby the potentiometer wire is wound on an insulated mandrel and thereafter a mixture of adhesive and solvent is sprayed over the wound mandrel [52] Cl 1;? from one side of the mandrel. By controlling the 1 Int Cl 6/ B32) 3 spray, the adhesive may be limited to approximately a c s s s a s s s s s I u I i l s a I "R. [58] f 156/6172 1 17/6105 potentiometer wire to the mandrel over approximately 1117/ PE 1 l 118/ 3 3 half the mandrel area while leaving clean and uncov- 2 2/703 53 61 23 ered the areas of the mandrel which will become the 1 9 wiper track and the areas in which end connections and taps will be attached. The spraying process may [56] References cued be accomplished as a separate operation after winding UNITED STATES PATENTS the potentiometer wire on the mandrel, or as a contin- 3,251,708 5/1966 Schmetterer et a1. 117/ 105.3 uous process by an addition of the apparatus of the in- 2,061,107 11/1936 schellenger 117/ 105.3 vention to a suitable winding machine. The apparatus 3,568,638 [833C- i of the invention adapts a conventional artists air brush n 4 131 83 for use as a sprayer and provides all adjustments and ac e a 3,007,828 1 H1961 Boyer et aL I I I I 161/184 controls necessary to achieve the desired result. 2,534,994 12/1950 Scott et al 156/172 7 Claims, 9 Drawing Figures 44 L '2: 44 bug/ 1 KPH! 1 '1 f! I! METHOD AND APPARATUS FOR MAKING POTENTIOMETER WINDINGS BACKGROUND OF THE INVENTION wound on a mandrel typically on the order of 0.050 I inches in diameter. The potentiometer wire is normally wound with a lead slightly exceeding the diameter of the potentiometer wire so that each turn is spaced a small distance away from adjacent turns, thereby avoiding undesired electrical contact between windings on the mandrel. By way of example, a potentiometer wire of 0.001 inches in diameter might be wound with a lead of 0.001 3 inches, thereby leaving approximately 0.0003 inches between each wire.

Excessive spacing between turns unduly increases the length of the wound mandrel required to yield a given resistance in the potentiometer wire wound thereon and, further, increases the apparent roughness of the surface over which the potentiometer ,wiper must travel, thereby tending to encourage wiper bounce and intermittant opens or noise as the wiper is moved along the potentiometer winding. On the other hand, if the turns are placed too close together, the slightest change in position of a given turn may result in contact between that turn and an adjacent turn, thereby shorting out or creating an undesired leakage path between the two turns and destroying the otherwise substantially linear variation in resistance along the wiper track in the finished potentiometer.

The common practice in the prior art is to wind the potentiometer wire onto the mandrel and then apply an adhesive or varnish to one side of the mandrel along the length of the mandrel so as to secure a portion of each turn to the mandrel, leaving the area which ultimately will become the wiper track in the finished potentiometer free of adhesive so that electrical contact between the wiper and the potentiometer wire will be assured. This adhesive is typically applied to the mandrel by hand after winding, essentially by painting the adhesive on one side of the mandrel over the length of the man-' drel by a brush or other applicator. Such a process is time consuming and the results of the hand operation very non-uniform. characteristically, the person applying the adhesive is looking down on a horizontally disposed mandrel and is applying adhesive to the upper surface. If excessive adhesive is used, or the fluidity of the adhesive is too high, the adhesive may run over the surface of the wound mandrel toward the lower surface, thereby coating the potentiometer wires in the area which was to become the wiper track, ultimately insulating that part of the potentiometer winding from the wiper and causing a failure in the finished potentiometer. On the other hand, if the adhesive used is too viscous or does not flow at all, the cemented surface of the winding will be rough and non-uniform, thereby interfering with the proper placement and alignment of the winding in a potentiometer assembly.

Another prior art method of applying adhesive to a wound mandrel is to draw the mandrel through an appropriate U-shaped channel while delivering a controlled quantity of the adhesive to the bottom of the channel. In this manner, the lower portion of the mandrel is wetted by the adhesive in the channel, the amount of adhesive being controlled by the rate at which the mandrel moves through the channel and the rate at which adhesive is delivered to the bottom of the channel. Because of the nature of this process, the adhesive used must be a relatively inviscous fluid. Consequently, the adhesive tends to flow around the mandrel and up into the area which will become the wiper track as a result of the capillary action between winding turns. Thus, this method also tends to yield a high rejection rate in potentiometer windings because of the presence of the adhesive on the wiper track, which insulates the wiper from the winding.

BRIEF SUMMARY OF THE INVENTION A method and apparatus for making potentiometer windings whereby the potentiometer wire is wound on an insulated mandrel and thereafter a mixture of adhesive and solvent is sprayed over the wound mandrel from one side of the mandrel. The mixture of solvent and adhesive is varied, depending on the relative velocity between the mandrel and the sprayer, and the thickness of adhesive coating desired, though in general a high ratio of solvent to adhesive is used to assure the achievement of a well atomized spray. By using a highly volatile solvent, the solvent evaporates shortly after impinging on the mandrel, leaving a thin coating of adhesive. Typically, an epoxy adhesive is used which has a sufficient viscosity to prevent migration of the adhesive around the mandrel so as to leave the back of the mandrel free of adhesive. To assure the overspray does not settle on the back of the mandrel and to remove the solvent fumes, a vacuum exhaust is provided under the mandrel in line with the spray to immediately remove the fumes and overspray.

The apparatus of this invention adapts a conventional artists air brush for use as a sprayer and provides all adjustments and controls necessary to achieve the desired result. Controls are provided for controlling the distance between the mandrel and the sprayer, to more accurately direct the spray toward the mandrel, to vary the airflow and to vary the quantity of spray. In addition, there is a provision for rotating the sprayer around the mandrel axis so that the direction of the spray with respect to the wound mandrel may be adjusted. In this manner, the portion of the surface of the mandrel to be coated may be chosen with respect to the mandrel cross-section, or for round mandrels the area to be sprayed may be chosen in cooperation with the location of a coiling head on other apparatus attached to the winding machine.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a top view of a typical potentiometer winding, with a wiper shown in phantom, presented to identify the elements and surfaces of the winding.

FIG. 2 is a perspective view of a prior art winding machine incorporating the spray apparatus of the present invention.

FIG. 3 is a front view of the spray apparatus, on an expanded scale, on the winding machine of FIG. 2.

FIG. 4 is a partial cross-sectional view of the spray apparatus of FIG. 3, taken along lines 4-4 of that figure.

FIG. 5 is a partial cross-sectional view, on an expanded scale, of the trigger 100 as shown in FIG. 4, showing the trigger construction in further detail.

FIG. 6 is a partial cross-sectional view of the spray apparatus of FIG. 3 taken along lines 66 of that figure.

FIG. 7 is a cross-sectional view of the spray apparatus of FIG. 3, taken along lines 77 of FIG. 4.

FIG. 8 is a cross-sectional view of the sprayer of FIG. 3, taken along lines 88 of that figure.

FIG. 9 is a cross-sectional view of the wound mandrel 64 illustrating the impingement of the spray on the wound mandrel and the resulting coating of adhesive.

DETAILED DESCRIPTION OF THE INVENTION First referring to FIG. 1, a top view ofa typical potentiometer winding may be seen. The winding consists of a resistance wire 30 approximately 0.001 inches in diameter, on an insulated mandrel 32 having a diameter approximately 0.050 inches in diameter. Subsequent to winding, the mandrel 32 is cut to length and bent to the circular shape shown. Leads 34 as well as additional taps, if desired, are attached to the winding by soldering or welding, generally on the inner periphery of the winding.

When mounted in a potentiometer, electrical contact is made to the potentiometer winding by contact in the form of wiper 36, shown in phantom in FIG. 1. This wiper contacts the top of each coil of resistance wire 30 and defines a wiper track around the upper surface of the coil. It is, therefore, important that this upper surface be free of all contaminants and, in particular, be free of all adhesives used to secure the resistance wire 30 to the mandrel 32. However, it is also necessary that the resistance wire 30 be secured to the mandrel 32 so that each coil or resistance wire 30 will remain in place, the ends of the resistance wire will not unravel, and the mandrel 32 may be bent into the circular shape without allowing the coils in the resistance wire to slide together or to be otherwise distrubed on the mandrel. Consequently, it is common to apply an adhesive in an area around the mandrel away from the portion of the mandrel which will become the wiper track and away from the area of connection of the taps and end connections. By way of example, the outer periphery 38 may have a layer of adhesive binding the resistance wire 30 to the mandrel 32 on the periphery of the ring, the adhesive being applied before the mandrel is bent into the ring shape.

Now referring to FIG. 2, a perspective view ofa prior art automatic winding machine incorporating the present invention may be seen. This machine incorporates the spraying apparatus of the present invention, generally indicated by the numeral 40, for practicing the method of the present invention. In this machine, a spool of mandrel wire 42 supplies a substantially continuous mandrel 44. The mandrel 44 first passes through straightening rollers 46, which eliminate the effect of having been coiled on spool 42. The mandrel 44 then passes through a flatening head 48, which rolls the mandrel into the cross-section shown in FIG. 9. The

mandrel then passes over roller 50, under roller 52 and over roller 54 to the winding head between rollers 54 and 56. Roller 52 is supported by a pivotally disposed arm 58 which maintains tension in mandrel 44, particularly in the length of mandrel 44 between rollers 54 and 56.

The winding head is located under shield 60 and winds a resistance wire on the insulated mandrel 44 with the desired lead as the mandrel 44 moves through the winding head at a uniform speed. The wound mandrel then passes around roller 56 and through the spray apparatus 40 which applies a thin coating of adhesive to one side of the mandrel. The mandrel then passes on through a precure oven 62 which partially cures the adhesive to secure the resistance wire for subsequent handling of the wound mandrel. The wound mandrel 64, after this precure, may either be cut into lengths for final curing and further processing, or may be passed through a coiling head 66 which coils the wound mandrel into a helical form substantially of the diameter desired for the finishing winding. The helical coils of wound mandrel are then postcured, cut to length and bent so as to substantially close the helix and create a ring shape as shown in FIG. 1.

Now referring to FIG. 3, a front view of the sprayer 40 on an expanded scale may be seen. The sprayer consists of a modified air brush 70 supported in a suitable mounting frame and disposed above the surface of the wound mandrel 64 which passes through suitable openings in the sprayer apparatus 40, as will be subsequently described, at a uniform rate of speed. In the preferred embodiment, the airbrush is one manufactured by Paasche Air Brush Co., and is modified in part by putting groove 69 in the lower portion of the brush and replacing the upper portions of the brush with member 71 containing groove 73. The air brush 70 is accurately located in air brush support member 76 by locating pins 78 engaging grooves 69 and 73 and is retained therein by thumb screw 80 securing retaining bar 82 over air brush 70. The air brush support member 76, shown in greater detail in FIGS. 4 and 7, is slidably mounted in cradle 77 and retained therein by strap 79. Adjustment screw 75, retained against axial motion with respect to cradle 77 by pin 81 engaging annular groove 83 in adjustment screw 75, threadably engages air brush support member 76 and may be used for vertical adjustment of the position of the air brush 70 with respect to wound mandrel 64.

Adjustment knob 84 passes through a clearance head 86 in the top of cradle 77 and is axially retained with respect to air brush support member 76 by pin 88 which engages annular groove 90 at the base of adjustment knob 84. A screw member 92 having a slot 94, engaged by set screw 96 extending through the back of air brush support member 76, threadably engages adjustment knob 84 so that the screw member 92 may be adjusted in vertical relationship with respect to air brush support member 76 by the adjustment knob. Adjacent to the lower end of screw member 92 is a conical depression 98 engaged by the extension of the trigger of the air brush, generally indicated by the numeral 100 in FIG. 4. This extension may be seen in greater detail in FIG. 5. A cylindrical member 102 is connected to the trigger of the air brush, and a conical protruding member 104 is slidably retained on the cylindrical member 102 by pin 106 inserted through slot 108 in the conical protruding member. A coil spring 110 elastically encourages the conical protrusion 104 into its fully extended position, which is more than adequate to extend to the bottom of depression 98 in screw member 92 as shown in FIG. 4. Therefore, the position of trigger 100 may be accurately adjusted by knob 84, and such adjustment will be maintained even upon the vertical adjustment of the air brush 70 by knob 108. As knob 84 is adjusted to adjust the trigger 100, the conical protrusion 104 is elastically urged into conical depression 98 as the trigger 100 is adjusted within its normal range, but when the trigger reaches the extreme on or the extreme off position, further motion of screw member 92 causes conical member 104 to compress coil spring 110 and to move out of conical depression 98. Thus, though an accurate control of trigger 100 may be accomplished by knob 84, the mechanical advantage inherent in the screw arrangement may not cause damage to the air brush 70 but instead will cause disengagement of the trigger operating mechanism from the screw member 92.

Cradle 77, which slidably supports air brush support member 76, extends below air brush 70 and is integrally attached to journal support member 112 FIGS. 3, 4 and 8) disposed below and laterally displaced from air brush 70. Hollow journals 114 pass through blocks 116 to engage mating bores in journal support members 112 and are locked in position by set screws 118 and 120. Thus, by loosening set screws 118 or set screws 120, the air brush 70 may be rotated about an axis substantially perpendicular to the axis of the air brush 70 so as to adjust the angle on the spray from air brush 70 incident upon the wound mandrel 74. A scale 122 (FIG. 3) is provided to facilitate accurate and repeatable adjustment of the angle.

The manner in which blocks 116 are supported by frame member 124 is shown in FIGS. 4 and 6. Internally threaded journals 126 engage mating holes in frame member 124 and are locked therein by set screws 128 in a forward projecting, substantially parallel disposition and extending into mating holes in blocks 116 so that these blocks may laterally slide thereon. A screw member 130 is axially restrained with respect to blocks 116 by set screws 132 which engage annular grooves 134 in screw member 130. Thus, the lateral position of blocks 116 with respect to frame member 124 may be adjusted by knobs 136 and may be locked at any desired position by knob screws 138, which are threaded through the sides of blocks 116 to engage internal members 126.

Now referring to FIG. 3, further features of the spray apparatus may be seen. Compressed air is delivered to the air brush 70 through hose 140, and may be regulated by a needle valve control generally indicated by the numeral 142. A mixture of adhesive and volatile solvent is delivered to air brush 70 through tubes 144 delivered from a tank generally indicated by the numeral 146, having a cap 148 and an on/off control 150. Also indicated in FIG. 3 is an outlet 152 for a gooseneck lamp (not shown), an on/off switch 154 for the lamp, and a provision 156 for supporting a magnifying glass (not shown) adjacent to the spray area.

It is apparent from the above description that the spray apparatus of the present invention includes provisions for adjustment of the spray and impingement of the spray on the wound mandrel without requiring any physical contact, other than the spray itself, with the wound mandrel. The adjustment provisions hereinbefore described include an adjustment of the separation between the air brush and the wound mandrel 64, the fore and aft adjustment of the spray with respect to the wound mandrel so as to assure direct spray impingement thereon, the angular adjustment of the air brush with respect to the mandrel, the control of the air supplied to the air brush and control of the volume of mix by control of the air brush trigger itself. In practice, it has been found that an accurate fore and aft location of the spray with respect to the mandrel 64 is desired in order to obtain highly repeatable spray impingement characteristics. To do this, a centerscope manufactured by Heco, Inc., Costa Mesa, California, has been modified as to have mounting surfaces duplicating those of air brush 741. Thus, the centerscope may first be mounted and used to center the spray axis with the mandrel axis before the air brush is mounted and the spray operation started.

In order to control the fumes and overspray, that is, the spray not directly impinging upon the wound mandrel, and to remove this spray from the vicinity of the mandrel so that it will not settle on undesired portions of the winding, exhaust devices are disposed around the mandrel. A vacuum hose 160 is disposed substantially coaxial with air brush 70 and below mandrel 64 so that overspray is immediately removed from the vicinity of the mandrel. However, because of the volatility of the solvent in the spray and the tendency of solvent fumes to rise, a sizeable exhaust hood 162 (FIG. 2) is disposed just above the sprayer to capture the fumes and prevent their passing into the surrounding room.

Having now described the apparatus of the present invention in detail, the method of use of the invention will now be described. In the automatic winding machine of FIG. 2, the mandrel 44 moves through the machine at a uniform speed (typically on the order of a foot a minute) and therefore, the spray apparatus 40 is attached directly to the winding apparatus by the rigid attachment of frame member 124 to the winding apparatus. It is to be understood, however, that the spray apparatus may be used with other winding machines by the appropriate mounting of the spray apparatus in relation thereto. By way of example, potentiometers are sometimes wound on machines (which may be modifications of commercially available metal lathes which simultaneously rotate the mandrel and advance with the desired lead the apparatus for delivering the resistance wire to the mandrel. The apparatus of the present invention may be used with such a machine by first winding the mandrel in the conventional manner, then stopping the mandrel and mechanically clamping the end of the winding, and finally traversing and spraying the length of the wound mandrel with the spray apparatus of the present invention. In such an application the provision for adjusting the angle between the mandrel and the sprayer is not required, because this angle is important it may be adjusted by setting the mandrel at the proper angle before spraying.

Now referring to FIG. 9, a cross-section of wound mandrel 64 showing the impingement of the spray therein may be seen. As previously described in rotation to FIG. 2, the mandrel 44 is flatened at flatening head 48, and after passing over rollers 50, 52, 54 and 56, moves past spray apparatus 40 in a substantially horizontal disposition. The spray apparatus is adjusted so that the spray impinges upon the wound mandrel at the desired angle and the'mixture of adhesive and volatile solvent is selected in conjunction with the air supply, and trigger setting so that the desired quantity of adhesive is deposited on the wound mandrel 64 as the mandrel passes through the spray area.

In general the spray apparatus will be adjusted so that the spray will impinge upon the wound mandrel 64 at an angle, and the coating of adhesive will be limited to the area of the mandrel within the direct line of sight of the air brush 70. By way of example, the spray 156 is shown in FIG. 9 impinging on the mandrel 64 at an angle of 17. Thus, when the mandrel 64 is coiled into the form shown in FIG. 1 with the lower surface 150 of the mandrel forming the inner surface of the coil, the region 152 of the mandrel 64, which will become the wiper track, will be free of the adhesive, while the region 154 adjacent the wiper track will have adhesive binding the resistance wire to the mandrel. Thus the resistance wire may be accurately cemented to and supported by the mandrel in an area adjacent the wiper track without risk of the adhesive covering any part of the wiper track.

When the apparatus of the present invention is used in conjunction with the winding apparatus of FIG. 2 the relatively low speed of the wound mandrel 64 past the sprayer apparatus 40 allows the use of an epoxy adhesive, thinned with a high percentage of volatile solvent. Typically the wound mandrel progresses past the spray apparatus at approximately inches per minute, and a mixture of Epoxy Resin adhesive, EC-229B manufactured by the 3M Company and Tetrahydrofuran mixed to proportions that will produce a mixture having a specific gravity of 0.894 at C, and sprayed under an air pressure of approximately 5 lbs, has been found to yield the desired result. The relatively high proportion of solvent to adhesive creates a well atomized spray, and the rapid evaporation of the solvent coupled with the relatively high viscosity of epoxy prevents significant migration of the epoxy around the periphery of the mandrel (this is also controlled in part by the separation between the spray nozzle and the mandrel, which typically is approximately l-l/l6 inches). Furthermore, the high ratio of solvent to adhesive allows the continuous spraying of the mandrel without depositing excessive amounts of adhesive to the mandrel, adhesive film thicknesses on the order of 0.002 inch being typical. Of course, air pressure, adhesive to solvent mix, and separation between sprayer and mandrel are interrelated variables, but each is easily adjusted to achieve adhesive film thickness desired and to control the flow of adhesive around the periphery of the mandrel.

While the invention has been particularly shown and described with reference to the preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the rit and scope of the invention.

I claim:

l. A method of securing a small diameter resistance wire wound on a larger mandrel so as to leave at least a part of each turn of resistance wire exposed to define a wiper track, said resistance wire and said mandrel forming a wound mandrel comprising:

spraying a mixture of adhesive and volatile solvent on said wound mandrel in a direction substantially perpendicular to said mandrel with a sprayer, as the relative motion of said wound mandrel and said sprayer in a direction parallel to said wound mandrel is maintained substantially constant and, curing said adhesive.

2. The method of claim 1 wherein a vacuum exhaust is provided for removing the fumes and overspray from around said mandrel.

3. The method of claim 1 wherein said adhesive is an epoxy.

4. The method of claim 2 wherein the volatile solvent is tetrahydrofuran.

5. The method of claim 2 wherein said vacuum exhaust is a first vacuum means disposed coaxially with and below the spray of said sprayer, with said wound mandrel extending through said spray between said sprayer and said first vacuum means, and a second vacuum means disposed above said sprayer.

6. A method of making potentiometer windings of the type having a small diameter resistance wire wound around an insulated mandrel and secured thereto so as to leave at least a part of each turn of resistance wire exposed to define a wiper track comprising the steps of:

a. winding said resistance wire around said mandrel so as to form a generally elongate winding thereon:

b. spraying a mixture of adhesive and volatile solvent along one side of said mandrel and over said winding thereon; and

c. curing said adhesive.

7. A method of making potentiometer windings of the type having a small diameter resistance wire wound around an insulated mandrel and secured thereto so as to leave at least a part of each turn of resistance wire exposed to define a wiper track comprising the steps of:

a. winding said resistance wire around said mandrel so as to form a generally elongate winding thereon;

b. providing a vacuum exhaust;

c. spraying a mixture of adhesive and volatile solvent toward said vacuum exhaust with said mandrel, in said spray;

d. causing substantially uniform relative motion between said mandrel and said spray in a direction substantially parallel to the axis of said mandrel, and

e. curing said adhesive. 

2. The method of claim 1 wherein a vacuum exhaust is provided for removing the fumes and overspray from around said mandrel.
 3. The method of claim 1 wherein said adhesive is an epoxy.
 4. The method of claim 2 wherein the volatile solvent is tetrahydrofuran.
 5. The method of claim 2 wherein said vacuum exhaust is a first vacuum means disposed coaxially with and below the spray of said sprayer, with said wound mandrel extending through said spray between said sprayer and said first vacuum means, and a second vacuum means disposed above said sprayer.
 6. A method of making potentiometer windings of the type having a small diameter resistance wire wound around an insulated mandrel and secured thereto so as to leave at least a part of each turn of resistance wire exposed to define a wiper track comprising the steps of: a. winding said resistance wire around said mandrel so as to form a generally elongate winding thereon: b. spraying a mixture of adhesive and volatile solvent along one side of said mandrel and over said winding thereon; and c. curing said adhesive.
 7. A method of making potentiometer windings of the type having a small diameter resistance wire wound around an insulated mandrel and secured thereto so as to leave at least a part of each turn of resistance wire exposed to define a wiper track comprising the steps of: a. winding said resistance wire around said mandrel so as to form a generally elongate winding thereon; b. providing a vacuum exhaust; c. spraying a mixture of adhesive and volatile solvent toward said vacuum exhaust with said mandrel, in said spray; d. causing substantially uniform relative motion between said mandrel and said spray in a direction substantially parallel to the axis of said mandrel, and e. curing said adhesive. 